This invention related to Internet Protocol-based voice communications, also known as voice over IP (VoIP), wherein telephone traffic is transported according to Internet-Protocol, that is using IP networks with packet switched communication. This invention is particularly useful in replacing conventional circuit-switched voice communication links.
Client/server model VoIP packet switched voice communication is known. An example is computer to computer VoIP using the Internet through high speed connections, such as a cable modem. Other examples are computer-to-telephone/telephone-to-computer configurations where outbound call routing is effected through plain old telephone services (POTS). Such configurations employ what is know as the H.323 protocol. The H.323 protocol is evidently limited in that it is not readily adaptable to handle incoming conventional telephone traffic.
A further example is a computer-to-telephone/telephone-to-computer configuration based on a standard protocol designated Session Initiated Protocol (SIP) SIP permits seamless POTS telephone-to-telephone calling via a direct interface connection or via routers and the like to high-speed cable and DSL portals into the Internet so that the voice path takes advantage of packet switched circuit efficiencies, such as use of a channel only when information is being transmitted and built-in error detection and correction. Such a routing control is ideally seamless to conventional telephone systems and private networks. Commercial products are emerging that employ SIP as the control path protocol. Recently, AT&T CallVantage was announced as a new service to augment the local and long distance services offered by AT&T. It has not yet been widely adopted.
FIG. 1 illustrates a conventional Voice over IP system 10. A SIP-to-public-switched telephone gateway provider 12 manages voice and control signals via logical paths 14 to and from the Internet, which is represented by an Internet cloud 16. Further logical paths 18 are coupled from the Internet cloud 16 to a Local Area Network (LAN) 20, where it typically must traverse a firewall 22 to provide logical paths 24 for voice and control to a Voice over IP (VoIP) client 26.
The SIP protocol has certain technical weaknesses which may prevent its widespread proliferation. Because SIP specifies use of UDP (User Datagram Protocol) port assignments that are nonstandard, namely, UDP ports 5060 and 5062, the SIP protocol cannot bridge the types of firewalls that protect many computer systems. More importantly, SIP protocol has a problem routing incoming calls through a firewall. This is known as a Network Access Traversal (NAT) procedure. The SIP protocol has become widely accepted, and these two SIP characteristics have been identified and have been used by some circuit switch-based telephone services to proactively block SIP ports and prevent IP traffic from using circuit-switched infrastructure to respond to incoming calls.
What is needed is a mechanism to allow VoIP to serve SIP controlled traffic through conventional firewalls and to permit both incoming and outgoing telephone traffic from conventional handsets.